The Cisco IOS Software Boot Sequence


Cisco routers perform the same types of tasks that a typical computer performs when you power it on the reboot (reload) it. Of course, most of us do not think about these details very often. The router performs some somewhat obvious steps, with one of those being tricky-namely, the process of choosing the location of the software to load and use in the router. And that software might not be IOS!

The boot process follows this basic litany:

1         The router performs a power-on self-test (POST) to discover and verify the hardware.

2         The router loads and runs bootstrap code from ROM.

3         The router founds the IOS or other software and loads it.

4         The router founds the configuration file and loads it into running config.

All routers attempt all four steps each time that the router is powered on or reloaded. The POST code

And functions cannot be changed by the router administrator. The location of the bootstrap code, the IOS to load, and the configuration file can be changed by the administrator-but you almost always use the default location for the bootstrap code (ROM) and for the initial configuration (NVRAM). So, the location of IOS or other software is the only part that typically is changed.

Three categories of operating systems can be loaded into the router:

  • The full-function IOS image.
  • A limited-function IOS that resides in ROM.
  • A different non-IOS operating system that is also stored in ROM,  and this can be loaded.

This operating system, called ROM Monitor, is used for two purposes-for low –level debugging and for password recovery. Unless you are performing password recovery, you would seldom use ROMMON mode.

Three OS Categories for Routers

Operating System Location Where it Is Stored Purpose
Full-featured IOS Typically in Flash memory can be on TFTP server Full-featured, normal IOS used in production.
Limited-function IOS ROM Basic IP connectivity, used when flash memory is broken and you need IP connectivity to copy a new IOS into Flash memory. Called RXBOOT mode.
ROMMON ROM Low-level debugging, usually by the Cisco TAC and for password recovery. Called ROM Monitor mode.

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